Conventional smoke sensors utilize light scattering or smoke ionization measurements, while conventional temperature sensors used in fire detection utilize thermocouple measurements. There are at least three disadvantages with conventional single sensor fire detectors: (1) there is a significant time delay between the start of the fire, and the transport of either combustion products or smoke to positions close enough to the detector to enable detection; (2) in instances when fire occurs in or at impermeable barriers (such as smoldering inside walls), the fire cannot be easily detected even in advanced stages of burning; and (3) single sensor detectors involve a high rate of false alarms due to any number of conditional changes in the operating environment, such as dust caused by vacuuming or smoke from burnt food, etc.
Combinations of smoke sensors and odor sensors which involve multiple fire signatures are less prone to false alarms (Okayama YU., Ito, T. and Sasaki, T., "Design of Neural Net to Detect Early Stages of Fire and Evaluation By Using Real Sensors' Data," In Proc. 4.sup.th Int. Symp. On Fire Safety Science, 1994, pp. 751-9). However, multiple sensors involve greater construction cost and increased complexity of signal processing hardware and software.
More recently, there has been increased interest in the use of radiation emission sensors (flame detectors) as an alternative to smoke and heat sensors (Middleton, J. F., "Flame Detectors," 9.sup.th Int. Conf. On Automat. Fire Detection, AUBE-89, Duisburg, Germany, 1989, pp. 143-54). The three major advantages of emission sensors are: (1) their ability to survey the entire room for fire initiation, (2) their fast response time, and (3) false signals can be readily distinguished since most fires are unsteady with unique frequency content, leading to unambiguous discrimination based on the power spectral density of the measured intensities (Grosshandler, W. L., "An Assessment of Technologies for Advanced Fire Detection," Heat and Mass Transfer in Fire and Combustion Systems, Vol. HTD-223, ASME, New York, 1992, pp. 1-9).
Single channel flame detectors operate either in the ultraviolet (where solar radiation is totally absorbed by the earth's atmosphere) or in the infrared (where flame emission is primarily from hot CO.sub.2) parts of the spectrum. Ultraviolet signals from flames are normally very low leading to false alarms from indoor radiation sources such as incandescent lights, arc welding processes, etc. Therefore, ultraviolet flame sensors are limited to outdoor usage where interfering solar radiation is absorbed by the earth's atmosphere. Another disadvantage of ultraviolet flame detectors is that even minute contamination of the optical windows causes a significant loss of sensitivity.
Therefore, infrared flame detectors are used for large indoor areas such as aircraft hangars and warehouses where direct solar radiation is minimal. Single channel infrared detectors look for radiation emitted from hot CO.sub.2 gases present in most flames at wavelengths around 2.7 .mu.m or 4.4 .mu.m. These single channel detectors must have precise band-pass optical filters in order to detect fires while successfully rejecting solar radiation. A corresponding problem with single channel detection is that since only one channel of information is present, the chances of false alarms are relatively high (Middleton, 1989; Okayama, et al. 1994).
False alarm problems present with single channel detection can be partially alleviated by using two channels of detection information. Typically two-channel flame detectors use one channel in the infra-red (typically at 4.4 .mu.m) to detect hot combustion products. The second channel is chosen above or below the 4.4 .mu.m band where there is a high level of solar radiation coupled with low levels of flame radiation. The addition of the second channel is purely for the prevention of false alarms by rejecting interference (such as direct solar radiation) from a continuum source that does not have the ubiquitous 4.4 .mu.m CO.sub.2 band. Fire is still detected using the 4.4 .mu.m infrared channel, and in cases where fire is present along with the interfering source, it might be difficult to resolve the signal unambiguously (Middleton, 1989).
Conventionally, frequency analysis of radiation from flames is complicated for at least two reasons. (1) Most pool flames in a stable undisturbed pool have a characteristic flicker frequency associated with them. This characteristic frequency depends on the diameter of the pool. For example, a 5 cm flame pool might have a flicker frequency of 6.7 Hz and a 1 meter pool of the same material might have a flicker frequency of only 1.5 Hz. Therefore, the flicker frequency can vary from 1 Hz to 20 Hz in many fires. (2) For forced flames (such as those issuing from a burst pipe or oil well), it is very difficult to distinguish any flicker frequencies. In such an instance, the frequency spectrum can be very flat from 0 to 15 Hz with no discernible flicker frequency.
Apparatus for such a frequency analysis is mentioned by U.S. Pat. No. 5,594,421 to Thulliard. However, the analysis requires input data over a long period (typically 10 seconds) to obtain a "discernible" frequency component. Jet flames and smoldering flames cannot provide this component.
U.S. Pat. No. 4,665,390 to Kern, et al. looks at a probability density function (PDF) of specific radiation intensities and compares that factor with a factor obtained from a Gaussian noise factor. This analysis is not a frequency spectrum analysis (or a flame flicker frequency analysis). One should note that even bright sunlight can have a non-Gaussian PDF of intensities. In fact PDFs and frequency spectrums are two non-overlapping aspects of any signal, and U.S. Pat. No. 4,665,390 utilizes the mean and Kurtosis (which are moments of the PDF) that have no relation to the frequency spectrum. Moreover, many different signals have the same PDF but different power spectral densities (PSDs). For example, a sine wave at 10 Hz and another sine wave at 20 Hz have the same PDF but totally different PSDs.
It is noted that both the above detectors, in addition to requiring a wait-state, cannot detect smoldering fires and/or operate in locations with fire places.
Conventional techniques have long utilized fiber optic for the transmission of sensed information over distance. Such apparatus is shown by Kern et al. in U.S. Pat. No. 4,701,624 and 5,051,590.